Guillermo Díaz (born March 4, 1985) is a Puerto Rican professional basketball player for the Capitanes de Arecibo of the Baloncesto Superior Nacional.

At the University of Miami, he finished 10th on the school's all-time scoring list with 1,477 points, including 174 three-point field goals.

In February 2008, he signed for Pepsi Caserta, a team in the LegADue, now known as Serie A2 Basket, the second division of Italian professional basketball.

After his departing from the Czech national league, Diaz announced that he was interested in including his name to the list of players in the Puerto Rican BSN draft.

He was involved in a movement that was trying to convince the team owners of the league to avoid the implementation of a salary cap to first-year players.

However, the team owners decided to put in cap on salary of twenty five thousand dollars, which led Diaz to remove his name from the list of players.